Electric-discharge machining which is conventionally referred to as EDM has been performed by generating a voltage between an electrode and a workpiece through a dielectric fluid such that a spark momentarily generated removes a portion of the workpiece surface to perform machining. EDM has more recently evolved into electric-discharge texturing (EDT) wherein the surface of a workpiece is textured. For example, rolls used in steel mills to roll steel that is to be painted cannot be too smooth or the paint adhesion will be improper. Texturing of the rolls provides the rolled steel with a textured surface to which paint adheres better. This texturing is performed by an EDT apparatus wherein a head having a bank of electrodes traverses the roll surface as the roll rotates. Actuators move the electrodes towards and away from the roll so as to achieve the correct gap between the electrodes and the roll. The actuators ensure that the electrodes are properly spaced with respect to the roll surface so as to be close enough to generate momentary sparks through dielectric fluid, but not too close so as to generate a continuous current flow without any sparks to perform the texturing operation.
A problem with a typical EDT apparatus is that the actuators are controlled with the same gain irrespective of whether the electrodes are to be moved toward or away from the roll. Using the same gain equally weights open and short circuited voltage conditions at the interface between the electrodes and the roll interface. This is disadvantageous because the difference in magnitude between the voltage associated with an open circuit condition and a discharge voltage is typically much larger than the difference in magnitude between the voltage associated with a short circuit condition and the discharge voltage. The discharge voltage is the voltage between the electrodes and the roll when sparking occurs.
Another problem with a typical EDT apparatus is that the spark periods during discharge are inconsistent. This occurs because the power switching control in the typical EDT apparatus switches on and off the power supply with a constant pulse train. Using a constant pulse train does not take into account that the time between pulse application and spark initiation varies depending upon operating conditions. In essence, the resultant spark period is variable depending upon the period between pulse application and spark initiation and a typical EDT apparatus does not account for this variability. The uneven spark period is a source particularly troublesome in EDT where consistent surface finishes are sought.
A further problem with a typical EDT apparatus is that of haphazard sequencing between power channels. An EDT apparatus includes numerous independently operating power supplies. To maintain consistency, the power output current must be the same across all power supplies. This is traditionally accomplished by generating a central on-off time control pulse train and distributing it to all the power supplies. This haphazard sequencing causes back electromotive forces to be generated as the power system is fully loaded at one time instant and then has no load at all at another time instant.